The present invention relates to a wander generator for generating a clock signal including wander. More specifically, the present invention relates to a wander generator having a TDEV mask characteristic setting apparatus for setting arbitrary TDEV mask characteristic which defines a time deviation (TDEV: Time DEViation) of a wander in an outputted clock signal.
As is well known, it is considered that a resistance characteristic against a phase fluctuation in a signal which is inputted in various electric devices and various transmission apparatuses, as one transmission characteristic of various electric devices and various transmission apparatuses which are incorporated into a digital transmission system.
Among these phase fluctuations in the inputted signal, a component of which fluctuation frequency is higher than 10 Hz is generally referred to as a jitter.
Alternatively, among these phase fluctuations, a component of which fluctuation frequency is lower than 10 Hz is generally referred to as a wander, which is described in the present invention.
Among these phase fluctuations, a time deviation (TDEV) characteristic is defined as an index for quantitatively evaluating a wander included in the inputted signal.
In the case of measuring this TDEV characteristic, a phase including a wander of a measured signal and a reference signal including no wander are sampled with a predetermined sampling frequency τs.
Then, a phase of the measured signal including the wander is compared with a phase of the reference signal including no wander, the phase difference TIE (Time Interval Error) at each sampling time is sequentially obtained as a change amount xi (TIE data) with respect to an initial phase difference, namely, the phase difference originally exists between both of signals.
Alternatively, this change amount xi at each sampling time is obtained from the following expression (1) as a power of the change amount xi at a time of respective integral period (integral time) τ(=n·τs) and it is defined as a TDEV characteristic (τ).                               TDEV          ⁡                      (            τ            )                          =                                            1                              6                ⁢                                                      n                    2                                    ⁡                                      (                                          N                      -                                              3                        ⁢                        n                                            +                      1                                        )                                                                        ⁢                                          ∑                                  j                  =                  1                                                  N                  -                                      3                    ⁢                    n                                    +                  1                                            ⁢                                                [                                                            ∑                                              i                        =                        j                                                                    n                        +                        j                        -                        1                                                              ⁢                                          (                                                                        x                                                      i                            +                                                          2                              ⁢                              n                                                                                                      -                                                  2                          ⁢                                                      x                                                          i                              -                              1                                                                                                      +                                                  x                          i                                                                    )                                                        ]                                2                                                                        (        1        )            n=1, 2, . . . , N/3 (integer)
In the above expression (1), xi means a change amount, N means a total of samples, τs means a sampling frequency, n means the number of sampling frequencies and τmeans an integral time (=n·τs).
In other words, this TDEV characteristic (τ) is obtained by evaluating a power spectrum density distribution of the phase difference by a time (lapsed time).
There are the following three methods (1) to (3) as a measuring method for measuring a characteristic with respect to a wander belonged to a measured system of the aforementioned various electric devices and various transmission apparatuses.
(1) Measurement of a Characteristic of a Wander which is Generated by the Measured System Itself
This is a method to obtain the aforementioned TDEV characteristic by measuring a wander included in the measured signal which is outputted from the measured system itself without applying an inputted signal, by using a wander measuring device.
Such a measured system comprises, for example, an oscillator or the like.
(2) Measurement of an Anti-wander Characteristic Belonged to the Measured System
According to this method, at first, a test signal including a wander having a predetermined TDEV characteristic (τ) is generated from a wander generator to be applied to the measured system. Then, a bit error measuring device measures the bit error in a signal outputted from the measured system.
In this way, it is evaluated how much this measured system can perform data processing without error with respect to the test signal.
(3) Measurement of a Transmission Characteristic of a Wander Belonged to the Measured System
According to this method, at first, a test signal including a wander having a predetermined TDEV characteristic (τ) is generated from a wander generator to be applied to the measured system. Then, the aforementioned TDEV characteristic (τ) is obtained by measuring the wander included in an outputted signal of the measured system by using the wander measuring device.
As these measured systems, for example, a transponder, an amplifier and the like are considered.
In the respective measurements of (2) and (3) among these three measurements, a wander generator capable of generating a test signal including a wander having a predetermined TDEV characteristic (τ) is needed.
As such a wander generator, for example, a wander generator constructed as shown in FIG. 9 is conventionally know.
In the other words, according to this wander generator, a modulation signal for modulating a frequency which is lower than 10 Hz and outputted from a modulation signal generation section 1, and a reference signal of a direct current which is outputted from a reference voltage generation section 2 are added in an adding section 3.
Then, by inputting an output of the adding section 3 in a voltage control oscillator (VCO) 4, the VCO 4 generates a clock signal CK which has a center frequency corresponding to the reference voltage and of which phase is modulated by the modulation signal.
This wander generator enables a frequency and a magnitude of the outputted clock signal CK to be changed by changing the frequency and the amplitude of the modulation signal which is outputted from the modulation signal generation section 1.
In the recent years, measuring standards for measuring a characteristic with respect to a wander belonged to the measures system are proposed by ANSI, ITU-T, ETSI or the like.
According to this standard, a TDEV characteristic (τ) including a wander outputted from the wander generator is defined by a standard value (a characteristic value) referred to as a TDEV mask characteristic so that a universality of the measuring result with respect to the measured system can be secured.
Plural kinds of these TDEV mask characteristics are prepared corresponding to, for example, the kinds of the measured systems and the measurement purposes. However, as shown in FIGS. 10A and 10B, a plurality of line segments 6a, 6b and 6c are connected, so that one TDEV mask characteristic 5 is constituted
Alternatively, a horizontal axis in this TDEV mask characteristic 5 is the aforementioned integral time τ(=n·τs) indicating the time elapse. A vertical axis is a characteristic value for indicating a power spectrum density of the change amount of a phase.
For example, the TDEV mask characteristic 5 shown in FIG. 10A is constituted of two line segments 6a and 6b. This TDEV mask characteristic 5 is defined in the following expression (2).TDEV(τ)=A1τ, τ1<τ≦τ2  (2)
Here, A0, A1, τ0, τ1, τ2 are respective parameters for specifying line segments 6a and 6b, which constitute this TDEV mask characteristic 5.
Specifically, τ0 and τ1 represent start points of respective line segments 6a and 6b, τ2 represents an end point of the line segment 6b, A0 represents a characteristic value at a start point τ0 and A1 represents an inclination of the line segment 6b. 
Alternatively, the TDEV mask characteristic 5 shown in FIG. 10B is constituted of three line segments 6a, 6b, 6c and this TDEV mask characteristic 5 is defined in the following expression (3).TDEV(τ)=A0, τ0<τ≦τ1TDEV(τ)=A1τ, τ1<τ≦τ2TDEV(τ)=A2, τ2<τ≦τ3  (3)
Here, A0, A1, A2, τ0, τ1, τ2, τ3 are respective parameters for specifying the line segments 6a, 6b, 6c which constitute the TDEV mask characteristics 5.
Specifically, τ0, τ1, τ2 represent start points of the respective line segments 6a, 6b, 6c, τ3 represents an end point of the line segment 6c, A0 represents a characteristic at a start point τ0, A1 represents an inclination of the line segment 6b and A2 represents a characteristic value of the line segment 6c at a start point τ2.
However, the wander generator shown in FIG. 6 only can modulate a phase of a single signal, so that it is difficult to generate a test signal including a wander of a TDEV characteristic, which satisfies the TDEV mask characteristic 5 constituted of a plurality of line segments 6a, 6b, 6c, of which characteristic values are different every for an integral time range determined by the aforementioned standard.
In order to solve such a disadvantage, the wander generator, in which a TDEV characteristic setting apparatus is incorporated, is developed.
In this TDEV characteristic setting apparatus, a plurality of TDEV mask characteristics, which are determined by the aforementioned respective standards, are stored in advance.
Specifically, the respective parameters A0, A1, A2, . . . , τ0, τ1, τ2, τ3 . . . are stored to specify a plurality of line segments 6a, 6b, 6c which constitute the respective TDEV mask characteristics 5.
When an operator of the wander generator inputs a characteristic number (graph number) for specifying the TDEV mask characteristic 5 on an operating panel, the respective parameters A0, A1, A2, . . . , τ0, τ1, τ2, τ3 of the TDEV mask characteristics 5 of the designated characteristic number which are stored in the TDEV characteristic setting apparatus are read out. After that, the TDEV mask characteristic 5 is formed to be applied to the modulation signal (fluctuation signal) generation section 1.
This modulation signal (fluctuation signal) generation section 1 generates a modulation signal (fluctuation signal) corresponding to this TDEV mask characteristic 5 on the basis of the inputted TDEV mask characteristic 5.
The modulation signal (fluctuation signal) corresponding to this TDEV mask characteristic 5 generated from this modulation signal (fluctuation signal) generation section 1 is added with a direct current reference voltage which is outputted from the reference voltage generation section 2 shown in FIG. 9, in the adding section 3.
Then, the output from the adding section 3 is inputted in a VCO 4 shown in FIG. 9, so that a clock signal CK which has a center frequency corresponding to the reference voltage, of which phase is modulated by the modulation signal (fluctuation signal) and which includes a wander having a TDEV characteristic (τ) corresponding to the designated TDEV mask characteristic 5, is generated from the VCO 4.
However, there is a problem even in the wander generator in which the aforementioned TDEV characteristic setting apparatus is incorporated, as follows.
In other words, in this TDEV characteristic setting apparatus which is incorporated in the wander generator, several kinds of the TDEV mask characteristics 5 which are determined by the respective measurement standards such as ANSI, ITU-T, ETSI or the like, are only stored as described above.
However, in recent years, a digital communication technology has made rapid progress, so that it is earnestly desired for the characteristic with respect to the wander not only to be measured as determined by the aforementioned respective measurement standards but also to be measured under various measurement conditions.
In this case, as a new measurement condition, it is desired a TDEV mask characteristic which is quietly different from respective TDEV mask characteristics 5 determined by the above respective standards is capable of being set as a matter of course, further, it is desired to set simply and for a short time the TDEV mask characteristics in which a portion of the respective TDEV mask characteristics which are determined by respective standards is only changed.